Abstract/Summary

The purpose of Research Theme 4 (RT4) was to advance
understanding of the basic science issues at the heart of the
ENSEMBLES project, focusing on the key processes that
govern climate variability and change, and that determine the
predictability of climate. Particular attention was given to
understanding linear and non-linear feedbacks that may lead to
climate surprises,and to understanding the factors that govern
the probability of extreme events. Improved understanding of
these issues will contribute significantly to the quantification
and reduction of uncertainty in seasonal to decadal predictions
and projections of climate change.
RT4 exploited the ENSEMBLES integrations (stream 1)
performed in RT2A as well as undertaking its own
experimentation to explore key processes within the climate
system. It was working at the cutting edge of problems related
to climate feedbacks, the interaction between climate variability
and climate change � especially how climate change pertains to
extreme events, and the predictability of the climate system on
a range of time-scales. The statisticalmethodologies developed
for extreme event analysis are new and state-of-the-art. The
RT4-coordinated experiments, which have been conducted with
six different atmospheric GCMs forced by common timeinvariant
sea surface temperature (SST) and sea-ice fields
(removing some sources of inter-model variability), are
designed to help to understand model uncertainty (rather than
scenario or initial condition uncertainty) in predictions of the
response to greenhouse-gas-induced warming. RT4 links
strongly with RT5 on the evaluation of the ENSEMBLES
prediction system and feeds back its results to RT1 to guide
improvements in the Earth system models and, through its
research on predictability, to steer the development of methods
for initialising the ensembles